Sorry to say, there are groups of ways you can lose access to your Windows logon account, one being easy forgetting the password. This happens ways more frequently than you would assume!

But in realism, there are numerous method you can end up losing access to an administrator account, with

1. Setting a fresh password for an Administrator account as well as after that forgetting the password.

2. Deleting an Administrator account and not being competent to log into any more one.

3. Altering an Administrator account to a standard user account plus not being capable to log into one more Administrator account